Amplitude stabilized sawtooth current generator



Dec. 25, 1962 M. FISCHMAN 3,070

AMPLITUDE STABILIZED SAWTOOTH CURRENT GENERATOR Filed May 7, 1959 2 Sheets-Sheet 1 --/9 zf T -5 b'ASEM/I. 5 1-200 2 3 8 -z -4 e cozucmk V0175 ATTORNEY 3,070,756 AMPLETUDE gTABILHZED SAWTOOTH (INT GENERATQR Martin Fischman, Wantagh, N.Y., assignor to Sylvania Electric Products line, a corporation of Delaware Filed May 7, 1959, Ser. No. 811,649 Qlaims. (Ci. 331-109) My present invention relates to a generator of electrical currents of sawtooth waveform.

In the generation of linear sawtooth currents in deflection coils that are primarily inductive, a problem arises in connection with maintaining the sawtooth amplitude independent of the sawtooth frequency. For example, when laboratory Oscilloscopes are supplied with deflecting currents from conventional sawtooth generators, undesirable changes in deflection of the oscilloscope beam take place at different frequencies, requiring time consuming readjustment or use of different scales and calibrations.

It is, therefore, an object of my present invention to provide a sawtooth current generator in which the peak to peak values of the sawtooth current waveforms generated are maintained constant over a range of operating frequencies. Another object of my invention is to provide a new, effective and relatively inexpensive amplitude stabilized sawtooth current generator.

Briefly, my invention makes use of a transistorized sawtooth generator connected to an inductive load such that currents of sawtooth form are caused to flow in the inductive load. By means of a current regulating transistor circuit, average current through the entire system is maintained constant. As a result, when changes in frequency are made, automatic, compensating voltage variations take plac across the inductive load in such a way that the peak to peak flow therethrough is independent of the frequency of operation.

My invention will be described in greater detail, hereinafter, with the aid of the accompanying drawing wherein:

FIG. 1 is a wiring diagram of an amplitude stabilized sawtooth generator according to my present invention;

FIG. 2 illustrates deflection generator waveforms and are used herein by way of explanation of the operation of my present invention; and

FIG. 3 is an illustration of characteristic curves of a transistor showing the relationship of collector current to collector voltage at different base currents.

In my invention, as illustrated in FIG. 1, currents of sawtooth form are caused to flow in'the inductive load here indicated as a coil 2, usually the deflecting yoke for a cathode ray tube. In accordance with my present invention the peak to peak value of this current flow through coil 2 is maintained constant over a desired operating range of sawtooth wave frequencies.

The sawtooth currents through the'load 2 are generated by th transistor generator comprising the transistor T having an emitter-electrode 4, a collector 6 and a base 8. Regeneration is provided by means of the autotransformer AT having its upper or high potential terminal 10 connected to the base 8 by way of the variable resistor R An intermediate point 12 of the autotransformer AT is connected by way of conductor 14 to the emitter of transistor T The lower terminal 16 of autotransformer AT is connected by way of conductor 18 and bypass condenser 20 Shunting the terminals 22, 26 of battery 24 is a bypass .condenser 25. The value of the voltage applied to collector 6 is adjusted to a suitable value by adjustment of the source voltage 24.

337,756 Patented Dec. 25, 1962 The condenser 30 is connected as shown between the base 8 and the conductor 18 which may be maintained at a low A.C. potential or grounded by the action of large bypassing condenser 19, insofar as the generated oscillations are concerned. Resistor 32 in series with the yoke 2 is a monitoring resistance from which a sampling voltage may be derived by way of lead 34 for test and adjustment purposes.

Transistor T serves to maintain the average current through the entire circuit constant. Transistor T is provided with an emitter electrode 40, a base electrode 42, and a collector 44. Emitter 40 is connected to ground through a resistor R thus providing degeneration in the emitter circuit. The voltage applied to the base 42 with respect to emitter 40 of transistor T is controlled by a tap 46, adjustably connected along and to resistor R Resistor R as shown, is in series with resistors R and R the series combination of resistors R R and R being connected between the negative terminal 22 of power supply 24 and ground.

If desired, modulating voltages such as voltages from a commercial 60 cycle source 53 may be introduced between the base 42 and emitter 40 by means of a transformer 50 connected as shown. The primary winding 51 of transformer 50 is connected to the 60 cycle source 53 and the secondary winding 52 of transformer 50 is, as illustrated, connected in series with base 42 and the tap 46 connected to resistor R The operation of my improved sawtooth generator may be followed and understood more readily by considering the system first, without reference to the amplitude stabilization feature. Referring to FIG. 1 (assuming point A to be grounded) it can be seen that transistor T provides regenerative feedback from emitter 4 to base 8. The feedback is of a magnitude sufficient to cause the transistor to operate in the saturation region of its characteristic. In this condition th transistor voltages remain practically in an equilibrium state due to the lack of dynamic gain of the transistor in the saturation region for an interval corresponding to the forward scan of the cycle. FIG- URES 2B, F, G, and H show this equilibrium state of the voltages on the transistor elements during this interval. The duration of this interval depends on the time required for the collector current, starting from zero current, to reach the sharp knee in the collector characteristic for the fixed base current that is applicable.

When the knee is reached the rate of change of collector current decreases and the transformer voltages start to drop resulting in decreased base drive. This process occurs quite rapidly due to the 'fact that the operating point is now out of the saturation region and the regenerative feedback of the circuit results in rapid cutoff of the transistor. The transformer voltage swings through approximately a half cycle of oscillation determined by the effective inductance and capacity of the circuit and the transistor loading including the internal resistance and capacitance of the transistor, whereupon the oscillation is arrested when the transistor turns-on and the next period of forward scan is initiated by the turnon of the transistor.

During the forward scan interval both junctions of the transistor are forward biased. Low voltage drops therefore exist across the junctions of the transistor resulting in an eflicient switch that connects the supply voltage --V across the deflection yoke 2 and the emitter tap 12 of the transformer AT. The base current or drive is determined by the voltage between the emiter 4 and the stepped-up end 10 of the transformer AT and the value of R the frequency control resistance. This resistance is high in comparison-to the base to emitter resistance of the transistor T under this operating condition. Dur- --ing the forward scan intervaL-the' base current-is fairly constant since the voltages referred to above, the emitter voltage and the voltage at the high end 10 of the autotransformer, are practically constant. The waveform of the base current of the transistor during the forward scan interval is shown in FIG. 2-1. With the 1:3 autotransformer used in the circuit of FIG. 1, a simple approxima- .tion for the base current useful for practical purposes is:

In the foregoing equation, E is the voltage across the lower section 12-16 of transformer AT, and is substantially equal, during saturation, to -V.

Estimated and measured values of base current at the end of .the forward scan interval indicate a value of approximately 4 ma. Referring now to the collector characteristics of FIG. 3, it can be seen that the transistor will be in the saturation region for collector currents up to 200 ma. with a 4 ma. base drive provided that the collector to emitter voltage is small. The emitter voltage waveform of FIG. 26 shows a .collector to emitter drop of about .4 volt near the end of the forward scan interval. FIG. 2E shows that the collector current increases to a maximum value of 200 ma. corresponding to the knee in the collector'characteristic for a base drive of 4 ma. The locus of the operating point of the transistor then falls out of the saturation region terminating the forward scan interval.

The duration of the forward scan may be estimated with reasonable accuracy provided some simplifying assumptions are made with regard to the circuit. The transistor and yoke inductance may be simplified to a series inductance-resistance LR circuit and the response to a step voltage E of 5 volts calculated. The total series resistance R is The parallel combination of the 1 millihenry yoke and transformer is 900 microhenrysl The collector curand the time required to reach a collector current .of 200 ma. may be calculated from =43 microseconds Referring duration of the forward scan interval as shown by the waveform is close to this value. The frequency of oscillation is conveniently controlled by the resistance R; in the base-emitter circuit. Increasing the value of R reduces the base current and therefore locates the knee in the collector characteristic at a reduced current. The period of forward scan is reduced in response to an increase of the frequency controlling resistance in the baseemitter circuit.

Undesirable changes in deflection with changes in frequency of the deflecting currents is due to the fact that the peak to peak current is a function of the saw-tooth duration in accordance with the approximation now to FIG. 2E, it can be seen that the the yoke inductance and T is the duration of the sawtooth or time of one cycle of the sawtooth waveform.

The average value of the sawtooth current drawn by transistor T is approximately I 2 if the ratio of trace to retrace time is large as is usualy the case. In accordance with my invention, I provide a transistor T to maintain the average current through the circuit constant. By maintaining the average current constant, the peak to peak current will likewise remain constant and desired stabilization ensues. Transistor T is connected with its emitter grounded through a resistor R emitter degeneration being provided by resistor R and the low base resistance of transistor T The high dynamic collector resistance thus obtained maintains the collector current practically independent of the collector voltage. Under these conditions the collector current of T will be fixed by its base bias permitting the voltage drop across the scanner circuit to vary in accordance with Formula 1 while maintaining the average current through the entire circuit constant. Thus for a given DC. current determined by the bias of transistor T adjustments of the frequency of the scanner will result in automatic compensating variations in the voltage E across yoke 2 with the result that I remains independent of the frequency of the scanner. For example, at higher frequencies the voltage across the yoke 2 will rise, thereby maintaining the peak to peak current through the yoke of unchanged value. The peak to peak scanning current will be independent of the characteristics of T the value of the yoke inductance, the sup-ply voltage V and will in fact be determined solely by the collector current of transistor T The stabilization of the collector current of T may be accomplished by emitter degeneration by the inclusion of resistor R in the emitter circuit. The potentiometer R serves as an amplitude control by establishing the DC. operating current in the collector circuit of transistor T and hence the peak to peak sawtooth current of transistOl T1.

Modulation of the sawtooth current can be accomplished by applying a modulating signal to the base circuit of T as explained, by use of a source 53 and transformer 50. The emitter degeneration serves to establish a linear relationship between the modulated sawtooth current and the modulating signal.

Table 1, below, indicates the amplitude variations as a function of frequency with and without the stabilization circuit and indicates a measure of improvement obtained with my present invention.

I claim as my invention:

1. In combination a first transistor having a base, an emitter and a collector; an autotransformer; a resistor connecting one end of said autotransformer to the base of said first transistor; a connection from the emitter of said first transistor to an intermediate point on said autotransformen'an inductor having one end connected to the emitter of said first transistor, means connecting the other end of said autotransformer to the other end of said inductor; a first condenser connected between the base of said first transistor and the other end of said autotransformer; a second condenser connected between the collector of said first transistor and the other end of said autotransformer; a unidirectional power supply; a second transistor having a base, an emitter and a collector; means for connecting the collector of said second transistor to the other end of said autotransformer, means for connecting said power supply between the collector of said first transistor and the emitter of said second transistor, and means for biasing the base of said second transistor with respect to its emitter so as to cause constant average current to flow through said first transistor whereby sawtooth current pulses flowing through said inductor will have a relatively constant peak to peak value over a range of operating frequencies.

2. In combination, a generator adapted for providing a sawtooth current to an inductive load, current regulating means coupled in series with said inductive load, and means coupling said series connected current regulating means and said inductive load across the output of said sawtooth generator, said current regulating means maintaining the average current through said inductive lo-ad constant.

3. A stabilized sawtooth generator comprising a transistor having an emitter, a base, and a collector, regenerative feedback means connected betwen the base and emitter of said transistor; an inductive load connected to the emitter of said transistor; current regulating means connected in series with said inductive load; and means adapted for connecting a voltage source between said current regulating means and the collector of said transistor.

4. A stabilized sawtooth generator as defined in claim 3 wherein said current regulating means comprise a second transistor having its collector-emitter circuit connected in series with said inductive load and further including biasing means coupled between the base and emitter of said second transistor, said biasing means controlling the average value of the current through said inductive load.

5. A stabilized sawtooth generator as defined in claim 4 wherein modulation means responsive to an applied signal is coupled to said biasing means, said modulation means modulating the current through said inductive load in accordance with said applied signal.

References Cited in the file of this patent UNITED STATES PATENTS 2,847,569 Finkelstein Aug. 12, 1958 2,891,192 Goodrich June 16, 1959 2,897,429 Jochems July 28, 1959 2,939,040 Isabeau May 31, 1960 OTHER REFERENCES Feedback-Stabilized Transistor Amplifier, by Slaughter in Electronics, May 1955, pages 174-175.

Electronics, by Herzog, May 1, 1959, pages 5253. 

